Sliding failure analysis of a gabion retaining wall at km 31+800 of Lubuk Selasih – Padang city border highway, Indonesia

, In August 2010, there was a landslide on the down-slope of national road section at Km 31+800 Lubuk Selasih – Padang City Border. In order to prevent further damage, it was necessary to make an immediate repair by constructing a gabion retaining wall. Since this repair was so urgent, physical and mechanical soil parameters for the stability analysis were determined from literature data. The stability analysis considered dangers of overturning, sliding, and soil bearing capacity. For the sliding stability analysis, the value for friction considered only the interaction between the soil and the base of the retaining wall, with the assumption that the contact area was equal to the total area of the entire base of the retaining wall. After the construction was completed, sliding failure occured due to pressure from the backfill embankment. This research performs a reanalysis of the retaining wall stability using soil and gabion parameters determined from field investigation and laboratory testing. In this reanalysis the friction contact area was assumed to be between the soil and the wire mesh of retaining wall. With these parameters and assumption, the main cause of sliding failure became clear, indicating that this approach increased the accuracy of stability analysis for gabion retaining walls.


1.Introduction
Instability of earth retaining walls is a serious problem in geotechnical engineering, with considerable potential damages on failure.Therefore, it is vital that the causes of instability be understood.
The failure of retaining walls has been extensively researched.A study in Vietnam of concrete retaining wall stability looked at sliding, rotation, and vertical deformation caused by fluctuations in groundwater [1].Instability caused by the construction failing to conform to the design based on prescriptive guidelines in a concrete retaining wall in India resulted in sliding, rotation, and vertical deformation [2].An investigation was performed into a 6m high gabion retaining wall in Canada that experienced vertical deformation and sliding three years after the construction was completed, and found the instability was caused by the lack of bearing capacity analysis [3].Stability analysis of a collapsed cantilever retaining wall in Lembah Anai, Indonesia found the lack of consideration of the dimensions in the design was the main factor contributing to the instability [4].Excessive deformation of a gabion retaining wall in Johannesburg, South Africa was investigated and the lack of bearing capacity analysis was found to the the main factor along with underestimates of lateral earth pressure [5].An investigation into a collapsed gabion retaining wall in Byreburnfoot,Scotland, and found the joints between basket couldn"t resist the lateral earth pressure from granular backfill [6].
This paper is a post failure analysis of a gabion retaining wall on a section of the Lubuk Selasih -Padang City border national highway at km 31+800 from Padang.In this case, the retaining wall experienced an excessive sliding failure just after the construction was completed.It showed no other sign of failure such as rotation and settlement.The excessive sliding showed that the retaining wall had a safety factor below than calaculated in the initial stability analysis, indicating the need for a reanalysis to find the causal factor for the instability.

Material
The material analysed included the backfill soil, foundation soil, and gabion stone fill, along with the type and specification of the gabion baskets used.

Soil and gabion stonefill
Physical and mechanical properties of soil were obtained from laboratory testing.Disturbed and undisturbed soil samples were collected collected from the site of the failed wall.
Physical and mechanical properties of soil and gabion stone fill are shown in Table 1 below: Table1.Properties of soil and gabion stone fill

Gabion basket
The gabion basket type used was fabricated from wire mesh with the properties from available manufacturer data (PT Jongka), as shown in Table 2 below: Table2.Jongka Gabion basket properties

Soil properties determination
Determination of soil properties was conducted in the laboratory and testing method for each property show in Table 3.

Gabion stone fill properties
In order to calculate the self weight of gabion retaining wall, a unit weight determination test was conducted by weighing a large box (0,45m x 0,45m x 0,60 m) filled with 15/20cm -20/30 cm angular andesite stone (approximately same size and density as gabion stone fill in field) and dividing by volume of box.

Field survey
A field survey determined the slope of the backfill surface, retaining wall geometry and deformation using triangulation from some benchmark points on the roadside and critical points on the top of gabion wall.It was found the top of the backfill on the backside of the wall had consolidated by an average of 30 cm, and this was useful to estimate initial unit weight of the back fill at initial condition.Measurement was also made of the the size and shape of the gabion wall cross-section.

Active lateral earth pressure of backfill soil
Active lateral earth pressure determinations were made based on Rankine's theory using the soil's mechanical properties as measured by the laboratory test and reviewed in initial and present condition.

Initial condition
Considering that sliding failure occurred within 12 hours of completion of construction and the backfill soil was in unconsolidated condition, active lateral earth pressure was analyzed in an unconsolidated and undrained condition [7]: In common practice, only active pressure distribution between z c and z = H is considered for total lateral active calculation, because there is no contact between soil and the wall in the tensile zone [7], active pressure distribution is shown in

Present condition
Active lateral earth pressure p h in present condition determined using effective shear strength parameter (Ø') and (c') [7] And active lateral force (Ph) is equal to [7]: Where: K a is coefficient of active lateral earth pressure is angle of backfill soil() ∅ is internal friction angle of backfil soil() P h is Total active lateral earth pressure, (t) c' is cohesion of backfill soil, ((t/m 2 )

Stability safety factor
Stability safety factor (SF) was examined for overturning, sliding, and bearing capacity and reviewed for initial and present condition.

Overturning stability SF
Initial condition [8]: Present condition [8]: Where: ΣFr is Sum of sliding resistance force (t) ΣPh is Sum of horizontal force.(t) The assumed sliding resistance force was that from friction between the gabion wire mesh at the base and foundation soil only assuming there is no direct contact between stone fill at the base and the foundation soil.The sliding force was assumed to be equal to the horizontal active force.
ΣF r [8] = ΣV tgδ (9) Where: is the friction angle between soil and the wire mesh at the base, for which the friction coefficient between soil and smooth metal proposed by [9] was adopted.

Soil bearing capacity SF
Soil bearing capacity (q u ) determined according to the equations used by Hansen (1971) and Vesic (1975) [8] and pressure on soil foundation (q) determined according to Terzaghi, 2002) [8].Where: if e > B/6 (12) and : Where: q u is ultimate bearing capacity of foundation soil (t/m 2 ) q is pressure on foundation soil (t/m 2 ) d c, d q ,d γ is depth factor i c ,i q ,i γ is load inclination factor N c ,N q ,N γ is bearing capacity factor D f is depth of foundation(m) Soil bearing capacit safety factor reviewed for initial and present condition.Foundation soil pressure is calculated based on active lateral force for each condition and foundation soil mechanical properties.

Flow chart
Research methodology is shown on flow chart below:

Soil physical properties (based on laboratory test result)
Physical properties for each soil sample was found to be as shown in Table 5 below: Soil classification was determined from grain size distribution and atterberg limit data with reference to USCS.Backfill soil classify as MH soil and foundation soil as GP-GM soil.

Gabion stonefill
Gabion stonefill consist of angular andesite stone with average size of 15/25 to 20/ 35 cm.Unit weight determination conducted by large box filled with andesite stone with approximately same stone size and density with gabion in the field and was found to be 1.722 t/m 3 .

Active lateral force
Active lateral force was determined for initial condition and present condition,and used equation ( 3) and ( 5) for calculation.Calculation results are shown in Table 7 below:

Stability safety factor
Stability safety factor was determined for initial and present condition for overturning, sliding and soil bearing capacity.

Overturning stability safety factor
Safety factor calculation follow eq(6) and ( 7) for each condition show in table 8 below:

Sliding stability safety factor
In sliding stability analysis, the sliding resistance force was calculated by eq (9), and the friction coefficient between foundation soil and gabion wire mesh was determined by an adopted friction coefficient (δ/Ø') of 0.4 between granular cohesive soil and smooth metal as proposed by [9].This meant the friction angle between wire mesh and foundation soil was 0.4x48.37=19,348º.
The Sliding stability safety factor for each condition is shown in Table 9 below:

Soil bearing capacity safety factor
Soil bearing capacity safety factor calculated by eq (10), (11), (12),and (13) for each condition and was as shown in Table .10 below:

Comparison of soil and stone fill mechanical properties
In table 11 below, we can see the comparison of the soil and stone fill mechanical properties between initial analysis (before construction) and reanalysis.In initial analysis, soil mechanical properties was refered to [10][11][12]  Note: Bf: backfill soil, f: foundation soil, sf: stone fill From table.11 above, we can see soil and stone fill mechanical properties in the initial analysis that was based on literature sources, were generally almost identical to the mechanical properties from the reanalysis based on laboratory test results excepting that in the initial analysis, undrained shear strength and soil mechanical properties for unconsolidated undrained condition (initial condition) was not determined.

Stability safety factor comparison
Comparison of the stability safety factors between initial analysis and reanalysis is shown in Table 12 below.In the initial analysis, active lateral force was determined based on effective shear strength of backfill soil alone and not calculated for the unconsolidated and undrained condition.This is considered a mistake because the clay soil was still in the undrained condition when the backfilling work was completed and a long time was needed to dissipate pore water pressure until inter grain contact with the soil was achieved, and effective shear strength of soil developed.So it was more appropriate to use undrained shear strength parameter in calculation.
In Table 12, the stability safety factor for each analysis is shown.For overturning stability, the safety factor has its greatest value in the present condition.This is because in the present condition the backfill has the lowest height and so cohesion of backfill soil has a bigger value than in the initial analysis making the lateral active force smaller.
In the initial condition, the overturning safety factor has smallest value as the active lateral force has the biggest value due to active pressure coefficient being equal to 1.
For sliding stability, reanalysis of the initial condition had the smallest safety factor and was lower than 1 because the friction coefficient in the initial condition had a smaller value than in initial analysis.Also, for the initial condition, the active pressure force had a larger value than in the other analysis.
The formula for the conventional sliding resistance force at the base of the retaining wall was taken from [8], where Σ = Σ and = 1/3 Ø' -2/3 Ø', and Ø'= internal friction angle of foundation soil.And if we use = 2/3 Ø'(as customary for masonry or concrete retaining walls) =32.247 º and Σ = 17.717 t/m' and SF sliding = 1,422 which is still less than the minimum requirement for the sliding safety factor .But we must consider, there is a difference between the interaction between soil and the base of the gabion retaining wall and between soil and the base of a stone masonry or concrete retaining wall.
Based on laboratory test results, it was found the internal friction angle of foundation soil had a larger value than in the initial analysis.In the present condition, the active lateral force is smaller, and this is the reason why the soil bearing capacity safety factor for the present condition is larger than in the other analyses.
All this demonstrates that the initial condition has lowest safety factor for all stability analyses and as it is lower than 1 or just 25.14 % of the initial analysis, this was why the gabion retaining wall was in an unstable state.

Fig 7a and 7b
show the difference in shape between the gabion wall before and after sliding and the resulting curvature.From table 4, we can see deformation of the gabion retaining wall varies from 0.49 m to 2.95 m from the Y-axis, and from -0.105 m to -1.65 m from the Xaxis.Field observation showed there was no broken wire in the gabion baskets due to this large deformation or sign of tilting of the gabion baskets.This along with the large deformation is further evidence that the gabion retaining wall was unstable in sliding stability.

Conclusion
Based on stability reanalysis for present and initial conditions and deformation measurement the following conclusions can be drawn: 1. Stability analysis in initial condition resulted in the lowest safety factor for all stabilities due to the largest active lateral force resulting from the unconsolidated and undrained condition of backfill soil.This emphasises the necessity of performing the stability analysis in the unconsolidated and undrained condition as the gabion retaining wall experienced excessive sliding deformation while the soil was still unconsolidated and undrained just 12 hours after the construction was completed.2. The sliding stability safety factor result from initial condition analysis is 0.792 or only 25.14% of the initial analysis indicating the gabion retaining wall was in an unstable state and liable to move.3. Sliding resistance force was overestimated in the initial analysis as the friction coefficient mistakenly assumed good contact had been developed over the whole area of the base as it would have been for concrete or masonry retaining wall.4. In the sliding stability analysis, it is more appropriate to use the friction coefficient between the gabion wire mesh and foundation soil only and assume no direct contact between gabion stone fill and the foundation soil especially when the foundation soil has a high bearing capacity. 5.If this considerations are applied in planning for future gabion stone fill projects the failure experienced in this instance could be avoided in future.
active lateral earth pressure at initial condition, (t/m 2 ) γ 0 is unit weight of backfill soil at initial condition(t/m 3 ) c u0 is undrained shear strength of backfill soil at initial condition,(t/m 2 ) z is depth (measured from the top of backfill) (m) Initial unit weight (γ 0 ) was estimated by multiplying the present unit weight (γ) by ratio of the present to the initial height of backfill (H/H 0 ), as illustrated in Fig 1.

Fig. 7b .Fig
Fig.7b.The gabion retaining wall after sliding 3.1.2Deformation Based on processed measurement data obtained from the field survey, the deformation is shown Fig 8 and Table4below:

Table 3
Properties determination method

Table 5
Table 6a and 6b, show soil mechanical properties based on laboratory test results also mechanical properties for backfill soil determined for initial and present conditions.

Table 6a .
Mechanical properties of the backfill soil

Table 6b .
Mechanical properties of the foundation soil

Table 7 .
Active lateral force

Table 8 .
Overturning safety factor

Table 9 .
Sliding safety factor

Table 10 .
Soil bearing capacity safety factor

Table 11 .
Soil mechanical properties comparison between initial analysis and reanalysis

Table 12 .
Stability safety factor comparison between initial analysis and reanalysis